The present invention relates to the art of pumps and, more particularly, to pumps suitable for use as heart or blood pumps or as ventricular assist devices. The invention is particularly applicable to pumps for the blood of a living person, or animal, intended to assume entirely or assist the pumping function of the biologic heart. It will be recognized, however, that the invention has wider application because it may generally be applied to environments that generally involve bearings, including hydrodynamic journal bearings.
There has been significant effort in the field of biomedical engineering to provide artificial blood pumps, i.e., non-biological devices that assist or assume entirely the pumping functions of the human heart. These devices are surgically introduced within the human cardiovascular system. Because of the unique biological environment in which they operate, blood pumps must satisfy very exacting operational requirements that relate primarily to the prevention of blood damage and the prevention of the loss of blood from the circulatory system. Of course, the dependability and longevity of operation of the device is also a major concern.
Blood pumps impart fluid motion to the blood that is not ordinarily experienced during normal biological processes. This fluid motion may jeopardize the integrity of the blood cells and present an excessive risk that hemolysis--the damage of blood cell membranes--will occur because of the excessive fluid shear and frictional forces experienced by the pumped blood. This risk is especially significant in small blood passages within the pump, typically including the fluid flow path between the pump impeller and housing. Blood cell integrity in the pumped blood is also at risk because of the heat generated by friction between the moving parts of the blood pump. Frictional energy may result ultimately in thrombosis--the undesirable clotting or coagulation of blood. It may also increase the potential for the formation of protein deposits within the pump structure or within the blood. To counter these effects, blood flow within the pump is often relied upon to provide a washing and cooling effect to the blood pump parts. In addition, blood has been used to lubricate the moving parts of the pump.
A blood pump that exemplifies the state of the art is disclosed in U.S. Pat. No. 5,342,177 to Golding et al, the subject matter of which is incorporated herein by reference. The invention disclosed therein provides a rotodynamic blood pump that utilizes a blood lubricated journal bearing between an annular rotor element, which includes an impeller for imparting axial motion to the blood, and a stationary bearing element, or stator, disposed inside the rotor. A driving means is disposed within the stator and is magnetically coupled to the rotor. The annular rotor cooperates with the axial extension to define two fluid passages. A primary fluid passage leads from the pump inlet to the outlet. A secondary passage provides a continuous flow thru otherwise stagnant areas of the pump. At least a portion of this second passage is narrowed to form a radial fluid bearing between the rotor and stator. In accordance with one aspect of the invention disclosed in U.S. Pat. No. 5,324,177, the axis of the drive element housed within the stator is deliberately offset from the rotor axis. In this way, a known magnetic force is provided to bias the rotor in opposition to the bearing fluid pressure forces and increased bearing stability is achieved. The use of blood flow for washing, cooling and, especially, the lubricating functions in journal bearings of state of the art blood pumps brings with it unique problems relating to blood integrity preservation.
Generally speaking, journal bearings are often used to support a rotating cylindrical member that is subject to a radial load. These bearings rely on a load-carrying film or cushion of lubricating fluid that resides between the rotating and stationary member on a side that is opposite the radial load. The operating principle behind a fluid film bearing is that the lubricating fluid is entrained by the journal into the load bearing film by the fluid viscosity. If the fluid passage is convergent in the direction of rotation of the bearing rotor, this action results in a pressure field in the load bearing film which provides sufficient force to float the journal and carry the radial load. As the passage converges, fluid pressure will increase. Conversely, if the passage diverges, fluid pressure will decrease and cavitation therefore becomes a concern in fluid bearing design. As a general rule, fluid bearing operation is characterized primarily by the viscosity of the lubricant, the speed of the bearing components, and the geometry of the bearing and, therefore, the geometry of the lubricating film. In addition to the foregoing constraints, journal bearings must be configured to prevent excessive vibrations that may develop during rotation and cause contact between moving parts and possible damage to the bearing over time. Minute imbalances in the rotating member may initiate vibrations as the member rotates. Without adequate stability in the journal bearing system, vibrations may become excessive, resulting in oscillating motions of the bearing parts at relatively large amplitudes compared to the bearing clearances. Instability is usually mitigated, at least in part, by the selection of a suitable lubricant.
Not surprisingly, the fluid and lubricating characteristics of blood are unlike those of conventional lubricants such as oil. The use of blood as a lubricant in a blood pump journal bearing therefore presents unique problems. On one hand, it is crucial that the journal bearing maintain the integrity of the blood flowing through it and permit enough flow to adequately wash and cool the component parts of the pump. On the other hand, the journal dimensions must not be such that the dynamic stability of the bearing is compromised.